Twilight switch

Presentation of Twilight switch

Another assembly that allows lighting a lamp at the dusk. It works in all or nothing, that is, the transition between the light bulb and bulb off is sudden and not gradual. Albeit by reading a little more …


No integrated circuit only the transistors, a power Darlington able drive a 12V bulb “good size”.


The principle of this scheme: 

An LDR (cell photo-resist LDR1) LDR03 type is associated with resistance and potentiometer (R1 + RV1) to form a resistive divider providing a voltage whose value depends on the illumination of the LDR. The RV1 adjusts the brightness threshold around which the bulb must turn on or off. The voltage from the divider bridge is directly applied to the base of transistor Q1. The absence of between divider series resistor and transistor base may well startle some, but rest assured, risk point of grilling by excessive transistor base current. Why ? On the one hand, because the transmitter of Q1 is not directly connected to the ground but through the resistor R 4, and also – and especially – because the value of the base current is directly related to the value of the resistor R1, which is high here. Just to get an idea (and be reassured if any), try to find the magnitude of the voltage and the base current of the transistor Q1. For that, see what happens in the two extreme cases, ie when the LDR cell is brightly lit and when it is in the dark.



LDR lit

The photocell has in this case a low ohmic resistivity of a few hundred ohms to several thousand ohms. So we can say that the voltage at node R1 / LDR1 is low, and that whatever the position of the RV1, since the total value R1 + RV1 is in all cases at least 180 ohms. Say to fix things, the voltage at this point may be a few mV. Under these conditions, it circulates in the base of Q1 a very low current, far too low to make its driver emitter-collector junction. The transistor Q1 is blocked, sees Q2 on its base a positive voltage reduced by the resistor R2, and becomes conductive. In doing so, it blocks transistor Q3. L1 lamp is off.

LDR in the dark

The photocell in this case has a very high ohmic resistivity, greater than 1 Mohm. This time, the voltage at node R1 / LDR1 step is sufficiently high to cause a base current in Q1, sufficient current to cause saturation. Q1 is saturated, transistor Q2 no longer receives on its base a sufficient voltage, since the base junction is found “more” grounded as to the positive pole of the power supply (R4 value is much lower than that of R2): Q2 crashes. Q2 is blocked, the base of Q3 receives sufficient voltage, through the resistors R3 and R5, to make it conductive. The light L1 lights.

Note: some particular arrangement of Q1 and Q2 provides a slight hysteresis, which means that the ignition threshold and the extinction threshold are slightly different. This prevents an effect of “flicker” when the ambient light turns around the switching threshold. The gap between the two thresholds is not phenomenal, but look for yourself the simplicity of installation. It should not be too difficult with this kind of circuit, do not you think?

Operating at reduced power

It is possible to operate this circuit with a supply voltage of 5 V instead of 12 V, the following arrangement is identical to the first except for the value of a few resistors.

Can be simpler …

Yes of course. One can simply two transistors, one view, as shown in the following diagram.

But try this circuit and compare it with the previous one, and get your own opinion. This second assembly fits you perfectly lit for the transition – off is very gradual and it is exactly what you were looking for? Well in this case, good for your wallet!

Selecting the photocell

Choosing the LDR (Light Dependent Resistor, light dependent resistor) was brought here on a model or LDR03 LDR05, but other models, a little more or a little less sensitive, are quite suitable. To be frank, I do not have not tried other than the above two, but I do not worry too much worries about it.

electronique_ldr_001 electronique_ldr_002

Two types of known LDR


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Post Author: gyoby

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